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United States Patent |
5,190,673
|
Leach
|
March 2, 1993
|
Filtration in solvent dewaxing with continuous spraying of filter with
both hot and cold solvents
Abstract
An improved solvent dewaxing process in which a method and apparatus for
continuous hot wash of a dewaxing filter is disclosed. A hot wash solvent
is continuously sprayed below the doctor blade followed by a cold solvent
spray below the hot solvent spray. Additionally, a solvent management
process for changing the proportions of the solvent in response to
different viscosity feedstocks thereby increasing filtration efficiency is
also disclosed.
Inventors:
|
Leach; Lyle A. (Morphett Vale Sth, AU)
|
Assignee:
|
Mobil Oil Corp. (Fairfax, VA)
|
Appl. No.:
|
714430 |
Filed:
|
June 12, 1991 |
Current U.S. Class: |
210/772; 208/33; 208/38; 210/784; 210/797 |
Intern'l Class: |
C10G 073/06; C10G 073/10 |
Field of Search: |
208/33,38
210/396,402,409,784,797,772,774
26/791
|
References Cited
U.S. Patent Documents
1874972 | Aug., 1932 | Hall | 210/772.
|
2040056 | May., 1936 | Manley | 208/38.
|
2048244 | Jul., 1936 | Bray et al. | 208/38.
|
2081296 | May., 1937 | Gard | 210/784.
|
3791525 | Feb., 1974 | Harris et al. | 210/217.
|
4820400 | Apr., 1989 | Ryan et al. | 208/33.
|
Foreign Patent Documents |
63-36809 | Feb., 1988 | JP | 210/791.
|
2023641 | Jan., 1980 | GB | 208/33.
|
8200029 | Jan., 1982 | WO | 208/38.
|
Other References
Hobson, G. D. (ed.), Modern Petroleum Technology, 4th ed., pp. 427-429
(Date Unknown).
Hengstebeck, R. J., Petroleum Processing Principles and Applications, pp.
256-257 (1959).
|
Primary Examiner: Drodge; Joseph W.
Attorney, Agent or Firm: McKillop; Alexander J., Santini; Dennis P., Cuomo; Lori F.
Claims
What is claimed is:
1. A process for continuously applying hot wash solvent to a rotary drum
filter used for solvent dewaxing and operated at a filtration temperature
in the range of from about -30 .degree. to about -5.degree. C., said
rotary drum filter having a doctor blade and a filter cloth, the
improvement which comprises:
discharging wax at said doctor blade;
continuously directing a spray of hot solvent below said doctor blade of
said rotary drum filter, said hot solvent having a temperature in the
range of from about 75.degree. to about 85.degree. C.;
continuously directing a cold solvent spray below said hot solvent spray to
cool said filter cloth back to the filtration temperature, wherein said
cold solvent spray is at about the filtration temperature.
2. The process of claim 1 wherein both said hot solvent and said cold
solvent comprises MEK.
3. The process of claim 1 wherein said solvent dewaxing is lube oil solvent
dewaxing.
4. The process of claim 1 wherein both said hot solvent and said cold
solvent comprises toluene.
5. The process of claim 1 wherein said hot solvent and said cold solvent
are sprayed concurrently.
6. The process of claim 1 wherein said filtration temperature is in the
range of from about -25.degree. to about -12.degree. C.
7. The process of claim 1 wherein both said hot solvent spray and said cold
solvent spray are at a pressure in the range of from about 50 to about 120
psig.
8. The process of claim 7 wherein said hot solvent spray and said cold
solvent spray are at a pressure in the range of from about 60 to about 75
psig.
9. The process of claim 1 wherein said filtration pressure is in the range
of from about 0 to about 5 psig.
10. The process of claim 9 wherein said filtration pressure is in the range
of from about 1 to about 3 psig.
Description
FIELD OF THE INVENTION
This invention relates to an improved process in which filter efficiency is
increased in solvent dewaxing units. In particular, the invention provides
a process for changing the proportions of the solvent and a method and
apparatus for continuous filter hot wash.
BACKGROUND OF THE INVENTION
MEK(methyl ethyl ketone) dewaxing is the process most widely used. In MEK
dewaxing, the wax bearing feed is mixed with solvent and the mixture is
chilled to crystallize the wax. The chilled feed is then filtered
continuously. Filtration is generally carried out in rotary drum filters.
The filtration zone is at the bottom of the drum with cold wash solvent
introduced at the top of the drum in the form of a spray to remove
occluded filtrate.
The solvent used in the process usually contains from about 45 percent to
about 75 percent MEK, with the remainder toluene. The concentration of MEK
is different for each feedstock depending on the viscosity of the feed.
The MEK component of the solvent induces wax precipitation while the
toluene component maintains the oil in solution. MEK dewaxing is described
in further detail in Hobson et al., Modern Petroleum Technology 427-429
(1975) and Hengstebeck, Petroleum Processing Principles and Applications
256-257 (1959).
Continuous filters are used in lube oil dewaxing. U.S. Pat. No. 3,791,525
to Harris et al. teaches a conventional rotary filter dewaxing apparatus.
The filtration rate generally declines as openings in the filter cloth plug
up with small wax and ice particles, requiring periodic washing with hot
solvent to remove the materials blocking the filter cloth. The filter must
be taken off stream approximately every eight hours in order to wash with
the hot solvent to restore the filter to the maximum filtration capacity.
No industrial dewaxing filters are known to be provided with continuous hot
wash capability. Therefore, it is an object of the present invention to
provide a method and apparatus for continuously applying a hot wash
solvent to a dewaxing filter in order to increase production capacity.
The filterability of the wax is also dependent on the viscosity of the
solution. Feed viscosity is different for each feedstock. A paraffin
distillate has a different viscosity than a light motor oil. Therefore, it
is a further object of the present invention to provide a method for
changing proportions of the solvent when processing different viscosity
feedstocks.
SUMMARY OF THE INVENTION
The present invention provides an improved solvent dewaxing process in
which the filter efficiency is increased. In a first embodiment the
invention relates to a method and apparatus for continuously applying hot
wash solvent to a filter cloth Continuous operation of the filter would
increase the production capacity of the dewaxing unit. The filter would
not have to be taken out of service on a regular basis. Further, the
filter would always be operating at the maximum possible filtration rate.
In a second embodiment the invention relates to a process for changing the
proportions of the solvent when processing different viscosity feedstocks.
The solvent composition if necessary can be changed as the feedstock is
changed and thus maximum efficiency may be achieved over a wide range of
operating conditions.
The invention therefore includes, in a first process aspect, a process for
continuously applying hot wash solvent to a rotary drum filter having a
doctor blade and filter cloth and used for lube oil dewaxing which
comprises:
discharging wax at said doctor blade;
directing a spray of hot solvent below said doctor blade; and
applying a cold solvent spray below said hot solvent spray to cool said
filter cloth back to filtration temperature.
In its apparatus aspects the invention comprises a continuous filter
apparatus for use in lube oil dewaxing said apparatus comprising:
a rotary drum filter, having a filter cloth;
a doctor blade engaging the filter cloth of said rotary drum filter;
an outlet for withdrawing wax at said doctor blade;
a conduit for directing a spray of hot solvent wash against the filter
cloth below said doctor blade; and
a conduit for applying a spray of cold solvent wash to the filter cloth
below the hot solvent wash conduit.
The invention provides in a second process aspect a process for changing
the composition of circulating solvent in an MEK dewaxing unit comprising
the steps of:
subjecting a petroleum feedstock to solvent dewaxing and thereafter
recovering solvent from both the dewaxed filtrate and the separated wax by
evaporation and stripping;
dehydrating the recovered solvent to form a dry solvent and recycling said
dry solvent to the dewaxing unit:
running a part of said dry solvent into a tank;
gradually splitting said dry solvent from the dry solvent tank into a MEK
fraction and toluene fraction and storing each fraction in a separate
tank; and
replacing the solvent volume routed to the dry solvent tank with MEK
recovered from the solvent splitter.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a simplified schematic diagram illustrating the filter apparatus
of the present invention.
FIG. 2 is a simplified schematic diagram illustrating the solvent
management process of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Feedstocks useful for the present solvent dewaxing process include
deasphalted vacuum resid and solvent extracted (furfural,
N-methyl-2-pyrrolidone (NMP) or phenol) vacuum distillates.
The dewaxing solvent is preferably a mixture of MEK and toluene. Other
suitable dewaxing solvents include, but are not limited to, methyl
isobutyl ketone (MIBK), acetone and propane.
The particular operating conditions used in the present process will depend
on the specific solvent, and will vary within the disclosed ranges
depending upon the available feedstock and the desired lube oil quality.
Process conditions such as temperature, pressure, space velocity and molar
ratio of reactants will affect the characteristics of the resulting lube
oil, and may be adjusted within the disclosed ranges with only minimal
trial and error by those skilled in the art.
TABLE 1
______________________________________
PROCESS CONDITIONS
Broad Preferred
______________________________________
Filtration temperature, .degree.C.
-30 to -5 -25 to -12
Filtration pressure, psig
0 to 5 1 to 3
Wash solvent pressure, psig
50 to 120 60 to 75
Viscosity of feed (solvent free),
2 to 35
Kinematic viscosity (KV),
centipoise (CS) @ 100.degree. C.
______________________________________
In a first embodiment the invention relates to a novel method and apparatus
for continuously applying hot wash solvent to a dewaxing filter. The
technique is based on applying hot solvent and cold solvent concurrently
to the filter. The use of the hot wash solvent during filtration avoids
the necessity of taking the filter out of service for approximately twenty
minutes every eight hours to remove the materials that plug the filter
cloth.
In the MEK process the waxes present in the oil feedstock are removed by
mixing the feedstock with a dual solvent consisting of MEK and toluene,
chilling the oil/solvent mixture and continuously filtering.
The solvent to feedstock ratio generally falls within the range of from
about 1.7 to 3.0.
The filter employed in the present invention is a rotary drum filter.
Rotary drum filters are described in detail in Kirk-Othmer Encvclopedia Of
Chemical Technology, volume 10, p.314-318.
A schematic of the overall configuration of the filter apparatus is shown
in FIG. 1. The oil/solvent mixture is continuously contacted in rotary
drum 1 with cold wash solvent spray 5 and dewaxed oil is continuously
discharged through port 6. Wax is discharged from the filter cloth 2 of
rotary drum 1 at the doctor blade 3. Below the doctor blade the filter
cloth 2 is subjected to a spray of hot wash solvent 4. Immediately
following the hot wash solvent spray 4 a cold wash solvent spray 5 is
applied to cool the filter cloth 2 back to the filtration temperature.
Cold wash solvent temperatures generally match filter feed temperatures,
falling within the range of from about -30 .degree. to about -5.degree. C.
The temperature of the hot wash solvent typically falls within the range
of from about 75.degree. to about 85.degree. C.
Continuous filter operation would increase production capacity of a
dewaxing unit since the filter would not have to be taken out of service.
The filter would always operate at the maximum possible filtration rate
which would represent a 10-15% increase in production for filtration rate
limited feeds.
The maximum MEK content of the solvent is different for each feedstock. In
order to optimize operation for each stock, it is necessary to have the
capability to remove water from the solvent, separate MEK from toluene,
and control the composition of the solvent circulated within the process.
A schematic of the overall configuration of the solvent management system
is shown in FIG. 2. Solvent is continuously circulated through the MEK
unit. The wet solvent recovered from the stripper and ketone tower
overheads from the MEK unit is dehydrated in the solvent dehydrator 11.
When it is desired to change the concentration of the solvent in the unit,
part of the dry solvent is run into a the dry solvent tank 12. The solvent
from the dry solvent tank is routed to the solvent splitter 13 where it is
split into relatively pure MEK and toluene streams and stored in the MEK
tank 15 and the toluene tank 14, respectively. The solvent volume routed
to the dry solvent tank is replaced with MEK from the MEK tank and sent to
the dry solvent accumulator 16 for use in the MEK dewaxing process. This
effects a rapid change in the solvent inventory composition.
The filter feed viscosity can be reduced by increasing the percentage of
MEK in the solvent thereby increasing the filtration rate. For example,
the percentage of MEK in the circulating solvent can be changed from 60%
MEK to 70% MEK, a about 4 to about 8 hours.
Another advantage of increasing the percentage of MEK is operation at
higher filter feed temperatures which further increases filtration rate by
reducing liquid viscosity. A further advantage of higher filter feed
temperatures is the use of less refrigeration, thereby increasing unit
capacity.
Another advantage of storing dry solvent in a tank followed by the gradual
splitting of the solvent is the maximization of MEK unit throughput with
minimum investment. Alternatively, a large solvent splitter could be used.
However, the large solvent splitter would require a more substantial
capital investment and only be in service a few hours a week. Another
alternative, slowly changing the solvent composition, would not allow
maximum operating time at the optimum solvent concentration.
Changes and modifications in the specifically described embodiments can be
carried out without departing from the scope of the invention which is
intended to be limited only by the scope of the appended claims.
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